1. Field of Endeavor
The present invention relates to the field of generator technology. It relates in particular to a static exciter system, and to a method for operation of an exciter system such as this.
2. Brief Description of the Related Art
Static exciter systems for feeding the field winding of a generator are widely used. They are distinguished by their robustness and a short response time. A static exciter system of the known type is shown in FIG. 1. The static exciter system 10 in FIG. 1 is associated with a generator 16, which is connected to a grid system (not shown in FIG. 1) via a (3-phase) busbar 19 and a machine transformer 11. The static exciter system 10 is used to feed the field winding 17 of the generator 16 and includes an exciter transformer 12, which is connected to the busbar 19 and whose output AC voltage is converted to a DC voltage by means of a thyristor bridge 18 which is fitted with thyristors 21. The thyristors 21 in the thyristor bridge 18 are driven, and the magnitude of the DC voltage is thus controlled, by means of an automatic voltage regulator (AVR) 14 on the basis of the generator voltage which is present on the busbar 19 that is tapped off via a voltage transformer 13, and of the current flowing in the excitation device 10 and measured via a current transformer 15. The DC voltage produced by the thyristor bridge 18 drives a field current If through the field winding 17.
The response of the exciter system 10 is increased by setting the output voltage of the exciter transformer 12 higher than the nominal level, so that it is possible to ensure the necessary current gradient dlf/dt (response time) and the necessary maximum value Ifmax (brief increase in the wattless component) by a temporary transition to full-wave control of the thyristor bridge 18. The corresponding ceiling factor, that is to say the ratio of the maximum field voltage to the rated field voltage, is in the range of 1.5 to 2.0. A limit value such as this is generally normal in order to cope with normal grid-system disturbances.
Despite this over-design, problems occur in conventional static exciter systems, for the following reasons: grid-system disturbances are frequently caused by short circuits in the transmission lines. Short circuits such as these reduce the voltage in the relatively close area of the grid system and in the nearby power stations. Since the exciter transformer is fed from the generator busbar, it likewise sees a reduced voltage. If automatic redosing (in short-circuit conditions) is successful within a predetermined time window, automatic reconnection to the grid system can take place. In order to guarantee the stability of the generator/grid system after reconnection, it would be desirable to introduce a brief wattless-component surge into the returning grid system. This requires the generator to be in an over-excited state. However, this can be achieved only partially because the voltage which remains during the short circuit and shortly after it does not allow a brief field-current surge to be built up. Even the ceiling factor that is incorporated cannot ensure adequate excitation in all circumstances during this time period.
The document U.S. Pat. No. 3,818,317 describes series feeding of a capacitor voltage to the analog signal loop of the exciter system. This feed is triggered by an under-voltage relay which samples the generator voltage. The signal modified in this way is applied to the input of the automatic voltage regulator (AVR). A brief increase in the power in the exciter circuit by feeding in from an additional energy source is not mentioned.
U.S. Pat. No. 6,339,316 describes on the one hand a capacitor-assisted uninterruptible power supply on the AC voltage side of the exciter supply (FIGS. 1-4), and on the other hand a capacitor which is charged with direct current and is connected in parallel with the supply line for the field winding (FIGS. 5-8). In both arrangements, the availability of the generator is adversely affected to a major extent, because complete excitation fails when one of the described components in the arrangements fails. The capacitor which is connected to the field winding uses a semiconductor switch to commutate the current away from the thyristor 10, and back again to the thyristor 10. This can lead to voltage spikes and to a malfunction or damage to the thyristors or the field winding of the generator.